I am a new pool owner (5-year-old pool came with house), and about 2 months ago I noticed faint brown stains on the sides and bottom. It's not bad, just not very nice looking. I bought the Jack's Magic Stain ID kit, and "Stain Solution #1" seemed to get the stain out just fine. The pool store guy said all I need is Metal Hold, which turns out to be HEDP. Since I read on a previous post that a small overdose of HEDP will precipitate unfilterable colloidal calcium phosphonate (and pool will look like milk to you drain it), I was hesitant to use this. But I am also hesitant to use the Jack's product, since I do not know what is in it; the MSDS was no help because it listed the components as a proprietary formulation. I don't want to add a lot of chemicals that could potentially make things worse. I have found your postings to be very helpful, thanks for any advice.
IG gunite; 15,300 gallons; cartridge filter; no spa; Houston, TX
CYA: 90 (I know this is high, have been working on filling and draining)